This can be accomplished with Natron’s “Retime” node. Keep your project frame-rate at 50 and place a retime node below your read node with the footage you want to retime. You’ll have to do a little bit of math to determine the speed value based on how many frames you want it to output. You should also set your project settings to the amount of frames you’ll be outputting so you can see them all on the timeline, in your example it is 750 frames which is 3.75x more than the input amount of frames.
Note that Natron’s retime node doesn’t do anything fancy to interpolate frames, it will just duplicate, ignore, or filter them as desired depending on the
Filter option selected. In other software you may have options for optical flow retiming or adding motion blur to retimed sequences using motion vectors which can sometimes generate better results.
If you’re also doing some VFX work on this footage I would recommend doing the actual retime last and working on the source footage to minimize problems that come with retime artifacts.