So I’m composing a song with “3” scales, G harmonic minor, A#/Bb Major and a set of notes that I can’t seem to find a scale for. C-D-D#-F#-G-A#-B I love how it sounds but don’t know much about music theory to modulate from this non existent scale I apparently made. It shares a lot of similar notes to the scales mentioned before and the song contains barely any chords, so it’s been causing me some frustration figuring out how to mod those initial scales to this one and back. Can anyone help me?
The general answer is that you can use one of the techniques listed in How many types of modulation are there?. You needn't think of your "non-existent" scale as anything special in terms of modulation. Any scale (or key), at core, is a collection of pitches, and changing from one set to another can employ any technique that suits the music.
More specifically, consider the three pitch collections involved, reordered to demonstrate common tones:
G harmonic minor: G A Bb C D Eb F# Bb major: G A Bb C D Eb F New scale: G Bb B C D Eb F#
All three scales are near identical, which makes common-tone / common-chord techniques a very straightforward possibility. G minor, A diminished, C minor, and Eb major are common to all of the scales.
Do you want your new scale to have a clear tonic? That is, do you want your scale to 'start on' C in particular? You should decide this before you write your modulation, because one of the main roles for the modulation is to establish the new tonic. You don't need to show the listener your whole new scale straight away - it can emerge naturally. In particular, that means you can use notes that appear in one scale right up until you finish your modulation into the next scale. You could do Bmaj-dmin-gmin-cmin, and the F and A in the dmin chord don't matter because they come before you've finished the modulation. (I know some trained musicians who think it's even fine to include notes from outside either the start key or the end key in a modulation, if the two keys aren't closely enough related).