Hmm…
Maybe you can do that, if you are familiar with changing values in Hex-view from SimPE. But I can't give you a complete instruction, because there are many things in the resources that I haven't decrypted.

As I remember right, the floors are stored in Instance '00' of '3D Array' resource. But I don't know which values you have to store for roads.
BTW: The offsets in my post above are not offsets for the values itself but describes the position in the files. Have a look at MTS2 Sims2Wiki:
http://www.sims2wiki.info

A great problem is that each resource of the lot uses his own coordinate system (shame Maxis for this). So for each resource we have to identify how to interpret the values. Also it depends from the value of 'U11' in 'Lot Description'.
For example the following graphic shows it for 'XOBJ' resource:


I don't know if this would help you, but I can't explain it better.
Sorry!