18 f/s tot 25 f/s

Yes, 18 fps is tricky with Premiere Pro.

As you've probably noticed, if you drag and drop a source 1080p18 ProRes 4444 XQ clip onto the New Item icon or to an empty Timeline panel to create a Sequence based on the clip settings, you'll get a Sequence with an 18 fps Time Base.  Still, it will not hold 18 fps when exported (the resulting file winds up at 15 fps).

Go ahead and edit in Premiere Pro to the point of getting to a fine cut, but hold off on any effects and color correction.  Send the resulting Sequence to After Effects for finishing and export using the After Effects Render Queue to get a 1080p18 ProRes 4444 HQ full-resolution edited movie.

Then use Shutter Encoder to encode to H264 or H265 if needed to maintain the 18 fps.

If I want to edit a digitized Super-8 film with an image size of 1920 x 1080 and a frame rate: 18.00 f/s in a HD project of 1920 x 1080 and a frame rate of 25 f/s

By @Studio Brandaris

What happens if you create a new Sequence in Premiere Pro @ 1920x1080, 25 fps and just drop your 18 fps footage there? If it looks good, edit the footage in a 25 fps timeline and export to 25 fps when done. Try it.