How do you weave a twill when the loom is threaded for a pattern?

You would need to analyze your threading and see if it can be adapted as a twill. For example, is it a summer and winter pattern? Or a crackle pattern? Lace? Something else? Once we see your threading we can maybe figure out a non-standard twill tie-up to get you something twill-ish. It depends on how regular the pattern is (with lots of repeats) or how irregular it is (no repetition). The second threading would be harder to get twill.