Splunk as such is not monitoring any particular equipment or solutions. Splunk ingests events (there can be ready-made add-ons for retrieving data from some types of sources) and can act on them.
So as long as you can provide splunk with the information regarding F5 certificates splunk can alert you if that information fulfills some conditions (like expiration data being in a near future). But the first thing to do is to get that information into splhnk.
As far as I can see, F5 add-on which you can install on your splunk infrastructure can pull some configuration data from F5 boxes but I have no idea if it contains certificate data. If it doesn't, you'd have to develop your own scripted or modular input that would pull that information from F5 or find a way for F5 to push that data to splunk (over syslog or other protocol).
