The indicators that we offer in Forex Tester are usually enough for any basic strategy. If you still need to add an indicator from MT4 that is not on our list, you need to convert it from .mql to .dll format.
The conversion is not very difficult because our API is very similar to MQL.
You can find API files in C:\ForexTester\Examples\Indicators folder.
Please see more information about API in Forex Tester → Help → Indicators API
Also, you can search for the indicators at Forex Tester forum, we provide many indicators there. As soon as you have the required indicator in .dll format, add it into Forex Tester via File → Install → Install New Indicator menu.