This is a limitation of the Planner, I am sorry to say. I suggest you manually enter the 10 most frequently used, then manually edit in the more rarely used ones into the plan after generating it. Does a single plan use more than 10 filters? If not you could put in the filters that are going to be used for that plan, then create the plan. Also note that you can create multiple observatory profiles and each can have 10 filters. So if you make plans that use one set of filters, then for some other mission you use different filters, you can create a setup with different 10-filter sets. Does this help? If not let me know here and we can set up a phone call to work out solutions.