It would be much better if you did not use the DVD as the “master footage” for the NTSC-to-PAL conversion.
The DVD is already severely compressed and will not give you the best results.
If you DO need to use the DVD as a master, you can convert the audio/video files back to an editable form by using DVD “ripping” software.
Popular Mac choices:
https://www.dvdxdv.com/
https://www.miraizon.com/products/products.html
https://www.alfanet.it/squared5/mpegstreamclip.html
But, again, if you can go back to the original edited footage that was used to create the DVD, that would be a better place to start.
Regardless, you will need to use a “standards convertor” to make the change in format.
In FCP, one of the most popular choices comes from Graeme Nattress.
https://www.nattress.com/standardsConversion.htm
Systems conversion always results in somewhat artifacted, lowered-quality images.
Start with the highest quality image you can.