Hi there and welcome to baby whisperer!
To answer your question, yes you can use PU/PD for naps only, daysleep is different to night sleep so you can sleep train day and nights separately.
Firstly though, could you post your baby's typical day in an EASY format e.g.
Wake up:7
E: 7
A: 2hrs
S: 9-9.30
Etc.
PU/PD is most effective when babies are doing a routine that suits their age, an overtired or undertired baby will take longer & be more difficult to get to sleep.
Secondly, have a read of this link..it's full of useful information on PU/PD..the why, when and how and frequently asked questions about it:
Pick Up/Put Down (PU/PD) - Everything you ever needed to know!Good luck, sleep training can be hard work but it's so worth the time and effort now for better sleep later xxx