This is likely fine. But, you should consider these two relevant items from the FAQ:
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Can I start working on the game before the jam starts? No. The game must be a new game project started and finished within the jam time frame, not just another week of work on an existing one. You are, however, strongly encouraged to have a working first-person grid-based view and movement set up ahead of time.
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Is it okay to start working on feature X before the jam? If it’s a feature useful in making any dungeon crawler (e.g. rendering, movement, interactable doors, general UI), then probably yes, but if it’s specific to the particular game you want to make, then probably no. One way to think of it: if it’s a feature generally already provided by another engine such as Unity or RPG Maker, implementing those features in your engine of choice is reasonable prep work. No. The game must be a new game project started and finished within the jam time frame, not just another week of work on an existing one. You are, however, strongly encouraged to have a working first-person grid-based view and movement set up ahead of time.
Note: I know many people who participate reuse / repurpose their previous combat systems each year. For example, I have a dice rolling system that I reused / cleaned up prior to the start of the jam last year. It is generic enough that it can fit in many games.