Can I use my 87 watt MBP charger for other things like portable batteries?
Yes, that's perfectly fine. Neither the battery nor the charger will be damaged by doing this.

Note that it won't be able to charge using the full 87W. The Powerstation XXL is limited to charging at 17W (5V at 3.4A).

The specification is in the manual here:

https://www.zagg.com/en_us/mwdownloads/download/link/id/483/

The manual also describes that charging from a standard USB-C charger (such as your MacBook Pro charger) is OK.