Depends on the site your planning on converting into Modx, im my experience anything is possible in modx and allot simpler.
If you have a knowledge of PHP then it will be allot easier for you as I have know knowledge of it and still manage thanks to this fantastic community
Here here Paul!
It might help if you posted the URL though, as people could then look at the site and comment on whether they notice any features that might need special treatment or consideration.
It’s good to share...
Mark
This might give you some idea of the steps you’d need to take:
http://bobsguides.com/porting-your-site-to-modx.html