I am still not living in Basel, but I have been doing some research this weekend.
I found out that there are several asociations of Spanish inmigrants.
In this one there is a 5-a-side football tournament which will happen 10and 11th of May. Unfortunately I will not be in Basel, but feel free to join.
These are the contact details.
I would be more interested in a 11-a-side football thing, but I will accept any proposals.