If the phone is unlocked, yes you can. You will need to buy a Straight Talk BYOP (Bring Your Own Phone) Kit. If the Galaxy is a GSM phone you will use the sim cards in the kit and you can switch in that way to Straight Talk but you will most likely not be on Verizon towers, but rather AT&T or T-Mobile depending on the region in which you live. Hope this helps.
if you are having problem once the switch to activate the phone. ##72786# that will activate your phone. I went through so much after the person I spoke to at Straight Talk told me I had to contact my carrier to unlock the phone. I even accidently erased everything out of my phone with no back up. Tried my luck again with Straight Talk and the guy at customer service told me how to activate the new service with this code. Happy I got my phone working but lost a lot in the process it all depends on who you talk to some know more than others
First I should say make sure to keep your sprint Sim card that supports 4G LTE that is what it is for.
I assume the sprint galaxy your referring to is the SPH-L720 or SPH-L720T, which would be the S4 model for sprint. I am currently in the process of trying to activate straight talk on my SPH-L720T model. My account from sprint it paid off and I did happen to use it on the MVNO Ring Plus for a month the service was excellent. Then I wanted to try Straight Talk Sprint BYOP and checked it said my phone is compatible. So I went through the hoops picked the correct zip code where the device will be used at basically where sprint has good coverage. Now I'm just having some minor trouble getting it to activate even though I paid straight talk already and cancelled my RingPlus Service which both use Sprint Network.
The only difference and thing why I might be having trouble getting straight talk to activate is because I always flash custom roms on my phone. But Ring Plus handled that with no problems. I have flash all the way back to stock to the newest Build.
FYI if push comes to shovel at one point I have my Sprint S4 using GSM straight talk service at one point in the past when I didnt have it paid off and it was still locked on Sprint, but I only owed a little so I paid it off. The way I had it on the GSM only was I flashed my model to MK5 it was OH1 which is a lollipop modem back to the kitkat modem I believe. I followed some methods to get the MSL number and the did the service mode thing to get to set it GSM and that worked for the time being then. Currently it has been switched back to CDMA and im completely stock, trying to activate the Straight Talk Sprint service. I hope it works too if not im going back to ring plus.