Follow Up Boss isn't a named import source, but you can still move your client records across. You export your client data from Follow Up Boss, then upload it to Marloo using Upload an export, and Marloo becomes where you keep your client records.
Go to Settings > Integrations. In the Import your client records banner, click Get started with a CRM import, then click Upload an export below the list of CRMs. The full steps, including what file types you can upload and what comes across, are in Move your clients to Marloo from another CRM.
CRM migration is free, and Marloo's team can move your data the same day.