Unless the PhotoMgrPro app has a feature that lets you export directly to Photos, you probably won't be able to do so on the iPad itself. The app should have methods to export their photos to a computer, or perhaps to a cloud service like DropBox. Your best bet is to do that, than then import from tht source back to Photos on the iPad.
I can't tell you about how to get photos out of PhotoMgrPro, but it's fairly easy to import from DropBox and a few other cloud services. There are also several apps for transferring photos between your computer and the iPad. I've always used Photosync by touchbyte for this and other large transfers of photos not easily done direclty from the iPad. That said, I haven't used it in a long while. Not since the newer iPadOS versions anyway.
*Note. There are several apps named Photoscync and similar. Be cautious and make sure you get the one from touchbyte, or do some research and get a photo transfer app you like better. Photosync works with iOS 9 and later, so should work with your older iPad.