With regards to the RFID question, I’m assuming you are talking about the Low Frequency 125kHz xEM side of your NExT.
If so, you are pretty much correct.
What I would reccomend is:-
If your security card admin people are willing to enroll you NExT into your system, that will be the most simple and fastest way to go.
If you scan your NExT on the reader/writer they use to enroll your current access cards and it successfully reads it, they just need to enroll it. JOB DONE
Depending on the access readers being used, you may need to play around with finding the "sweet "spot for reading.
If you have a DT 125 kHz X Field Detector this will help a lot to find it…
If they won’t or cant enroll it, you will need to confirm if the NExT is compatible with your security system you provided in the link to above:-
https://www.hidglobal.com/products/cards-and-credentials/indala/flexkey-keytag 5
Hopefully some of the Gurus on here can answer that for you.
If it looks like it is compatible and if you end up needing to Clone your current security tag, I would reccomend…
first go with @ebryer suggestion in the message above. Cheap chinese cloner and get a/some T5577 card(s) to test on first.
EDIT
actually probably HID cards
DT are out of stock of the cheap cloners, however in this link there is a lot of info and a video, worth a read/watch
https://dangerousthings.com/product/xem-cloner/
Failing that, if you want to invest in a Proxmark
https://dangerousthings.com/product/proxlf/
alternatively
Depending on where you live, there might be somebody on here willing to help you out with access to their knowledge AND Proxmark.
Good luck I hope you manage to get it to work.
If so please share back here, as there may be others in the same situation at some stage.