Can I pay social security payments per day/job for occasional work?

Hi,
I am a blogger/editor who works full time contracted for a media company in Spain (social security paid).

I would like to take on occasional freelance work for UK or American companies (maybe one project every three or four months, about two to three days worth of work). Obviously this is not worthwhile if I have to pay autonomo for a full month, but is there a daily rate or per job rate I could pay?

Thanks in advance for any reply.