You need an external current shunt.
What you do is basically connect your meter to read VOLTAGE across a short length of wire. As that wire carries a higher and higher current, the voltage drop will be measurable.
You will first need to calibrate it by using it to measure a known load. You might measure the current required for a light bulb. Say, it is 1,75A. Then you place your shunt in series with the load and measure the voltage across it (when 1.75A is presumably flowing through it. Let's say it measures 12mV.
Now you know that for each 12mV on your meter, you are drawing 1.75A. You could do some math to tell you that you read about 6.9mV per amp.
When connected in series with your real load, and you measure (say) 147mV, you can calculate the current at about 21.5A
If you're going to do this often, you would make adjustments to your shunt so that the conversion factor was simple (like 10mV per amp) to allow you to read the current almost directly from the meter.
