The Cottagers boss has already brought in Venezuela centre-back Fernando Amorebieta, Ghana midfielder Derek Boateng and Holland goalkeeper Maarten Stekelenburg on free transfers so far this summer.
Jol has been heavily linked with moves for Tottenham midfielder Tom Huddlestone and is expected to seal a swoop soon for Wolves winger Bakary Sako, with talks already confirmed by the League One club.
In addition, Jol could bring Cameroon international Eyong Enoh back to west London after the defensive midfielder impressed on loan from Ajax in the second half of last season.
Jol said: "We were quite early getting Amorebieta and the others - we are very happy to have them on board. It's always nice to integrate them, and take the new players away with us.
"We are still working on one or two - we want them going with us as well. There will be one or two coming back, because we had a few loan players. I'll try to get them in now, because 14 players are left and we're still working on five or six players to come to this club.
"Three are already here so that was good, because most of the time you do your business late. This was quite early, so that was a positive sign."
Fulham will play three pre-season friendlies against Costa Rican Primera Division clubs at the Estadio Nacional in the capital of San Jose.
Next Sunday, they meet last season's Verano championship runners-up Cartagines, followed by a midweek clash against Bryan Ruiz's former club Alajuelense who were Invierno champions in December.
The tour concludes with Fulham taking on the most successful club in Costa Rican football history, Deportivo Saprissa, on Saturday July 20.