You can use a constant interval in the style (.drt) file but add a blank stimulus to the end of each trial in your input file, like this:

stim = ~
loc = 1,1,1
time = 500

That would give you a 500ms ISI for that trial. For a 300ms ISI, you simply change it to:

stim = ~
loc = 1,1,1
time = 300

And so on. You can also have these values be random, or randomly chosen from a set. For that see, www.empirisoft.com/directrt/help/directrt_using_random_values.htm