If you are patch 1.67 with a disc install you can use
Easy CASP Editor to recategorize EA EP and SP clothing, and CC clothing installed via the launcher. It's not a mod but a separate program which creates a custom tuning mod with your changes. Doesn't work with Steam or patch 1.69.
Or you can use NRaas MasterController. From City Hall or computer>NRaas>MasterController>Settings>CAS>Disable Clothing Filter>Everyday. That will show all installed clothing in Everyday. Choose the swimsuit in MasterController's Stylist setting (you'll need MC's Integration module for this change to affect in-game plan outfit on the dresser). Then you can set the clothing filter back and I think your sim should keep their swimsuit as Everyday.
Or choose the swimsuit in the swimsuit category then click on the sim>NRaas>MasterController>Intermediate>Copy Clothing Between Categories>Choose the swimsuit category>Copy to Everyday. This will
add the swimsuit you chose to Everyday as a new outfit.