If you only buy one phone, the answer is no. After you have activated the phone with a new number, porting another number to that phone would amount to "bring your own device", whcih Xfinity mobile doesn't support yet.
However, through 9/13/17, you can buy two LG X Charge for $1 each (plus taxes and fees). You can then activate a new number on one and plan to port an old number to the other. If the phone doesn't work out, you are only out a few bucks (the actual amount will depend on where you are located). You will have to cancel the services though as I read Xfinity mobile charges monthly fees and such even for phones that have not been activated.