Yes. You can still list items that are fulfilled by you after joining Fulfilled by Souq program.
Souq Fulfillment Centers : Locations and Contact information
Listing and Creating FBS Shipping Notices
How can I register with Fulfilled By Souq?
Can I request to recall my Fulfilled by Souq products from your fulfillment centers?
Who handles customer service complaints and feedback and provides product support for Fulfilled by Souq products and orders?
How can I monitor numbers related to Fulfilled by Souq in the selling center?