If you've been without insurance for fewer than 63 days, any medical condition cannot be counted as pre-existing (per Health Insurance Portability and Accountability Act of 1996). Otherwise, you can still check with the company to see if your condition counts as "pre-existing" by their definition.
You checked that they are excluding it permanently, or is it just a set exclusion period (e.g. 6 months)? In the latter case, you can pay yourself for related treatments till the period ends.