How would you say: [not] trespassing on someone else's field of expertise
Solution 1:
A common colloquialism would be "treading on your toes" (though Xanne's comment works too).
You could also say "I know this is your area, but..."
"Crossing your specialty" sounds odd to [this] native English speaker, and "trespassing", while quite a nice way of putting it, would work better with a preposition ("trespassing in" or "trespassing on").