Galaxy S7 user with inactive iPhone 6 - Can I get iMessage with my active phone number? I have a pretty unique and specific scenario. 

Here's some context. My friend lost his iPhone 6 in my house last October. Since he had insurance on the phone, he was able to replace it. I ended up finding it four months later. To complicate matters, Sprint said the phone's IMEI is blacklisted because the phone is listed as lost or stolen. My friend is contacting Sprint to let them know that he has found the phone, and he will let me keep the phone since I found it.

I already have a Samsung Galaxy S7 with my active Sprint number (415 area code). I prefer to use my Samsung for daily use with that number. 

What I am looking to do is use the iPhone just for iMessage, so that I can still be in touch with friends and family who have iPhones while I'm out of the US. I only want to use the iPhone 6 to use iMessage when connected with Wi-Fi because I am going overseas for work in January. I might pause the active Sprint ph#(415) account while out of the US since I wont be able to receive SMS messages while in Europe and Asia but still considering all options.

Assuming my friend is able to get the phone's IMEI off the blacklist, will I be able to activate the iPhone with my Sprint phone number and get iMessage up and going, then go back to my Samsung Galaxy S7 once I get iMessage account running? Are their any easier ways to get my end goal of communicating thru iMessage without any obstacles?
Once you start using the iPhone on your account, and activate iMessage, you will no longer be able to receive SMS on the Galaxy from any contacts who use iPhones, because from Apple's perspective, your number will now be an iMessage number.

If you wait until you're ready to leave to do this, that will work. However, if you pause/cancel/whatever your service with Sprint, you will only be able to receive iMessages if the person has the email address associated with your Apple ID in your contact on their iPhone, and you have that email address set up for receiving in iMessage settings on your iPhone. If your service stays active, messages from other iPhone users will still be received by your phone via your phone number, but only if you're connected to Wi-Fi (or enable data roaming, which would probably be a bad idea if you like keeping your money).

Once you get back, assuming you want to resume using the Galaxy, you'll want to deregister your number from iMessage or you will back in the same predicament as far as not being able to trade messages with iPhone users.