1. The import tool relies on MS Graph API for filters. You can find some more information about how this works and how it can be configured here: https://learn.microsoft.com/en-us/graph/api/user-list?view=graph-rest-1.0&tabs=http#examples/. The import tool can only work with using user filters or group filters but not a combination of both. You would set which ones to be used in the "Search" parameter, with two possible values: "users" or "groups". In terms of how to configure a filter, I found this resource quite helpful, it allows one to try various queries, filters, etc: https://developer.microsoft.com/en-us/graph/graph-explorer
2. Since this is MS functionality we won't be able to provide much here apart from links to their documentation. However, based on my (not extensive) knowledge of MS, extensions show up in Microsoft Graph within the Extensions collection, they are not top-level properties. Our Entra import tool does not currently process extensions, but this is something that will be available in a future import tool update (possibly even next release).