No. You can't.
You can add new Eras in the CIV5Eras.xml file, no problem. But two things then break:
1> The tech tree's TechTree.lua is hard-coded to go from ID=0 to ID=6. (Search for "0, 6, 1".) The new era you add will be #7 by default, so you MUST change the .lua file if you want the tech tree to display. There's at least one other instance of hard-coding in that file that I have yet to find (as I'm trying to add new eras for my own mod).
(Note: this is only the DISPLAY of the tech tree. The techs themselves will still be where they're suppose to be, but you'll just get a blank screen where your tech tree used to be.)
Also, the era ID numbers need to go in chronological order. So if you decide you want to insert an era between Industrial and Modern, for instance, you'll need to manually renumber the ID codes of the Modern and Future eras to make room for your new one at ID#5.
2> Certain parts of the game seem to be hard-coded; for instance, the game looks for Future Tech as the final repeatable tech, so you can't just delete it. Likewise, I tried deleting the Future Era and reassigning the techs to one of my new eras and it didn't work too well. If you want to add a new era at the end of the tech tree, then, you'll need to do a LOT of rearranging to make sure every tech still has an era, every era still has at least one tech, the IDs are still in order, and the GridX values don't overlap between eras.