Converting to dng should only be considered a temporary solution. DNG is not anything close to a universal format outside of the Adobe world, so I would strongly advise against using it, and do not toss your original nef files.

Here are three points of view - against, it's ok if you have to, and it's the best thing since sliced bread:

https://photographylife.com/why-i-no-longer-convert-raw-files-to-dng

https://mattk.com/why-i-dont-convert-dng/

https://photographyconcentrate.com/should-you-convert-dng/

I don't convert to dng. And the only time I use dng is when I edit a file in DXO PhotoLab which exports an edited dng file back to Lightroom, and when I use Lightroom to create an HDR or Panoramic merge. So rather than edit individual files, I can edit the result of the merge as a dng raw file.

But something is troubling me. You say you are using Lightroom Classic. The only way you can get LR Classic is with the $10/mo subscription or the $20 one, which gives you the same LR/PS/Bridge/ACR/LR Mobile and adds one TB of cloud storage for your images. So you should be able to upgrade your software by signing into your Adobe Creative Cloud account.

If your Lightroom is properly installed, it should recognize your Photoshop and use it as it's default external processor. It is a simple matter to edit your images in LR, then use the Edit In command to open your raw files as layers in Photoshop, and that would get you to do what you want - merge to pano in PS.

For the record, the pano and HDR merge in Lightroom Classic are faster and better than the old way of opening the images as layers in PS.

On the other hand, if your troubles are because you are running stolen software - and I am not saying that you are - but if - then you've got some nerve posting here looking for support for it.

If you are legit - then accept my apologies for the rant.