Another way would be to use the "Options" menu, to enable only specific grade levels. That's pretty good for keeping in practice... but there can be up to 200 characters in a grade!
The most specific option is to put together a "usefile". The way to do that within jdrill, is to make use of the "usefile" checkbox, in the search window. Whenever you come across a particular character you want to repeat a lot, check that box. When you have 10 or more such characters of interest to you, go to the Options menu, and enable "usefile". The program will then pick quiz lines only from your list, instead of from the whole dictionary.
Pre-built usefiles, in groups of around 20 lines, can be downloaded here. You can download the whole lot as the ".jar" archive, or just grab a few to try out.
[ Notice that the top caption is red, not blue.] You can now see what lines you have selected, bring up any particular one in the search window, and uncheck the usefile box if you don't wish to see it again