Adding the Mount Ability Manually?

[Apoc_Golem]

Is there any way to add the Mount animal companion ability manually? There are several animal companions that I homebrew to allow to have the Mount ability because a) it's ridiculous that it doesn't have it in the first place (such as riding dogs and wolves), and b) certain class/archetype features will only allow you to choose a horse even if you're willing to ignore the loss of benefits from having the Mount ability. For example, a Small cavalier can't pick a wolf or riding dog as a mount, despite the fact that it makes absolute sense (particularly the riding dog; though it's not even listed as an option, you could theoretically just use the wolf stats).

I looked all over for a way to manually add the Mount ability to an animal companion but came up empty. The closest I came was the "Value Modification" Permanent Adjustment under the Adjustments tab, but that didn't actually add the Mount ability.

Am I just missing something, or do I just have to stare at an obnoxious error message every time I log into a riding character's sheet?