Adding objects to already built models! PLEASE HELP

[atognaccini]

If I were to detail another model by adding more features and more 3D elements (i.e. adding a cowcatcher to the front of a regular coach to make it look like a cabcar)---- what software should I use? And how would i be able to add a file to paint it and give it color? Please reply i'm in a desperate state of wanting superior realism :hehe:

Cheers
Alessandro