Not too hard to do. You'll need two main items.
(1) A set of extra test leads. They don't have to be anything special, just not defective. Once these are modified, they can only be used as current probes.
(2) A low value resistor that is physically large in size (one with a higher wattage). 0.1 ohm or 1.0 ohm is best, precision helps.
Splice into the middle of the test leads, and solder in add the resistor. This resistor is in parallel to the leads.
I chose 0.1 ohms, you'll see why.
Add some hot glue to strain-relief the wires then wrap in electrical tape. You're done! Now current can be measured with any volt meter.
Below you see a setup where I'm measuring current being provided by a current-limited power supply set to 0.5A. This particular multimeter doesn't measure DC current natively.
The current flows through the one pen, through the resistor and back to the supply through the other pen. That creates a current loop that doesn't require a voltmeter to be connected, but that is what we plan to do.
The other half of the test leads connected to the meter collects the voltage drop across the resistor we added. We become an observer to the resistor's voltage.
The voltage drop on the voltmeter is calculated by the simple equation I=V/R.
Since I chose a 0.1 ohm resistor, my equation is I = V / 0.1, which is the same as I = 10 * V by simple algebra. In another words, multiply the voltage read by the conductivity of the resistor we chose (Siemens=1/R) to get the current through the resistor. I multiply 0.0502 mV by 10 Siemens and get 0.502 Volts, but I replace the word Volts with Amperes.
Notice the total voltage drop at the power supply is 0.225 V, of which only 0.050 Vis across the resistor. This means my wires are about 0.175 ohms for the two wires leading up to the resistor. This value would be less if I didn't use cheap-o test leads. Even so, my meter is reporting the voltage across the resistor, not the entire circuit. The source voltage is ignored because it is irrelevant.
If you chose 1 ohm, you don't even need to multiply. The voltage reading is the same as current. However, the higher the resistance you choose, the more your setup will affect the circuit you are testing. Also, the more heat your resistor will have to withstand (needs to be larger). Your resistor wattage should be greater than Imax * R^2, where Imax is the largest current you want to measure.
Bottom line, when measuring current, you don't want to introduce too much resistance to the circuit. To measure current you have to "break into" one of the current-carrying wires to make your measurement. The less noticeable your interruption, the closer your current reading will be to the true value.
Now you can add these special probes to your toolbox where you keep your current-less meter. If you want to be extra careful, you could also add a fuse to one of the pen wires. Depending on the quality of your wires, you'll want to measure no more current than about 5 or 10A. If you choose to measure high currents, proceed at your own risk and keep your measurements short in duration, only keep things connected long enough to get a quick reading.
Truth be told, inside your multimeter there is typically a 0.01 ohm "shunt" resistor. Behind the scenes, the multimeter measures the voltage across that resistor and magnifies it 100 before reporting the current. So your setup is essentially doing the same thing as a ammeter does, you are simply doing it outside the multimeter.
Hope this helps some of you make do in a pinch, or be creative in one of your projects. You don't have to use pen tips, you could very well use some sort of clips to attach to the circuit.
Bonus: Did you know you can measure current through a resistor without breaking into the circuit? Find a resistor that is in the current path and measure the voltage across it while it operates. Get out a calculator. Type in the measured Volts, press divide (÷), then type in the Resistor value. The result = is the current in Amperes.
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