[FreezeBoxJawsh]

I've looked through the rules and I can't find a ruling on can I buy knowledge skills for things like "mechanic" or "construction" or horticulture post charagen. I also can't find how much it would cost to buy additional points once I get the knowledge skill added. Would It be treated like adding a +1 rating to body or agility and you spend the karma equal to 2x the level you are buying into

[ZeConster]

Attributes and Skill Tables, page 105-106, has a section on that: buying ranks in Knowledge and Language skills works the same as in active skills, except you only pay [new Rating] instead of [new Rating x 2] Karma to raise a Knowledge/Language skill 1 rank.

[FreezeBoxJawsh]

Excellent this works out so much in my favor haha thanks

[Tarislar]

I've looked through the rules and I can't find a ruling on can I buy knowledge skills for things like "mechanic" or "construction" or horticulture post charagen.

I'd point out that Mechanic is an Active Skill of the Engineering Group,  it also acts as its own Background-Knowledge skill.
Construction & Horticulture, IMHO, would also be active skills as well if you were actually building/growing something with them as well.