I assume you mean that you want to modify your existing rims offset? I think it may be a tough one. I have heard offsets being altered on steal rims but not on alloys. I know Simmons Wheels will make an alloy wheel to whatever specs and offset you want (very pricey though), however I don't think they will modify an existing rim. Perhaps give them or a similiar company a call and see what comes up.
Usually 3-5mm (depending on the wheel) can be shaved off the back plate (see orange part in image). Most wheel repair or machine places can do this for you. 3-5mm can make a world of difference to scrubbing