In the channel effects for your sound, you can apply Logic's AutoFilter plugin, which will allow you to cut off a frequency according to various wave shapes, timings, distortions, amounts, and all of this can be broken into x number of EQ bands. I believe it will have the effect you're looking for.
If you want to specifically control the LFO of a synth, pretty much each synth in logic has LFO controls in it. Since you're using Logic Pro X, I'll tell you, definitely start using Alchemy as both a synth and a sampler. You have tremendous filter controls in there and you can start out using it just like EXS24.