Mike, yes you can control working calenders at this level in P3.1. It does have the constraint that you can only choose 1 planning unit (Months, weeks, days or hours) but within that you can schedule differnt workday shifts and work days per week as well as holidays etc, all of which could be pressed into service as artificial duration extenders.

^ what Deiter says

also; if you want to manually add float to the programme, there are 4 ways to do it (I list in my order of preference, best first)

Wear the float on your sleeve
Add an activity inline with the critical / near critical path labled "float activity". The most honest way, but depending on your audience you may well want to filter these activities out of your presentation. This is my preferred method, as the gap is clearly labelled. you need to maintain control of these though, if discovered by the client they will be eaten like candy.

hide it in the activities
Increase the duration of the activity you want the float on. Sneakier, but still dangerous. A sharp eyed client will likely see that you don’t need 10 days to build that scaffold and argue the duration down. Pop goes your float

Constrain starts
Constrain the start date of a CP activity to be later than it needs to be, this will open up float. Dangerous and inflexible for the same reasons as adding lags is (see below)

hide it in the links
Add lags to your links to create gaps. Very dangerous as you’re likely to forget that they’re there adn there’s no way to notate them (in P3)