I don't have the game yet, but I'm contemplating it. One question we have is if it's possible to create your own character from scratch with your own career, etc, or if you have to use the pre-created careers/templates/characters with their displays.

According to the rules: no

Can it easily be done: yes

In my opinion the careers seem pretty basic and can easily be created with experienced group and a yes-man for a GM. There's the question about the career "speciality" and balancing the career. But I can not see why it still couldn't be done. No rules for such though as said.

I'm working on a squire card as we speak. Constructive input most welcome, here it is:

Squire.

Basic Career: Human.

Basic, Combat, Knight, Military, Social

Primary Characteristics
Toughness, Fellowship

Career Skills
Charm, Discipline, Leadership, Ride, Weaponskill

[C][C][R][R]

Talents* Tactic, Reputation.

Advances

Action 2
Talent 2
Skills 2
Fortune 1
Conservative 1
Reckless 1
Wound 1

You can make your own classes in D&D, it's also possible to make your own careers in WFRP. But there are so many already, and will be even more, when they begin making more and more expansions...