Modifying and Editing existing buildings

[juicestain 0]

Hello there, I've been using this site to download many mods to my game. I recently started wanting to add my own content and things of that nature and one of the things I really want to do is to modify some of the existing buildings to better suit the needs of the city I'm currently working on. I have scoured the site for answers but haven't been able to find an answer taylored to this specific conundrum. (Also, I'm quite new to modding.)

 

I have the Lot Editor and all dependents for that. I've created a copy of one existing building (small fire station), but I am unable to open it in the LEProp program (as it seems I can only open .dat files.) I know that's where I would implement the changes I wish to make, I just need a little help getting from Point A to Point B. Any direction in this would be helpful.