Question

[caksz]

Is it possible to mod siege engine to pass through city gate ?

[Nellup]

This belongs in the main modding forum, but I'll answer anyway.

I assume you mean Onagers, Scorpions etc.
AFAIK, no, although fiddling about with the mass and other stats might be able to make something happen, or scaling down the siege engines models perhaps. More than likely though, you'll break it.

Asking the main modding forum will generally get you better answers.