As far as I can remember, no, Garou can't put a permanent 6th+ dot in any Trait, even using experience - though I think it's just strongly implied, not explicitly stated (at least in the 2nd edition core rulebook, which we used.) What seems to support this is that five dots are considered "Superb" (in the introductory description of traits), and that the official character sheet simply has no more dots to fill.
However, you, as the Storyteller, can overrule this, of course.
Mind you, I don't really agree with the view that the bonuses resulting from the various forms are only temporary. A Garou - again, as far as I can remember our WW:tA gaming days :) - is all the other, non-human / non-wolf forms: he can remain in them indefinitely (even though that's rather impractical in a lot of situations), and they're part of his nature, his self, just like Homid. Gifts and other stuff are temporary, but shape-shifting is not - or, if you wish to see it that way, having a shifting, shape-wise temporary nature is part of what makes a werewolf. Homid and Lupus are also temporary, for you don't stay in them permanently (though you may get stuck in them for a real long while, if you're unlucky.) This balances out, imo, the limitations of the Homid form: your average vampire has to put quite a lot to raise a trait above five dots temporarily, whereas a Garou can boost himself above five in the first scene of your first story (and chances are he'll need it, sorely. :))
Also, do consider the name "Homid": it implies that in it, a werewolf seems, behaves, and works like a human. Anything else would reveal his true nature to the Sheep. The Homid form is for blending in, for taking on the sheep-skin - viewing it as "the real character" instead of just one facet of a Garou's shifting, five-fold nature is a distortion, a misperception, especially in-game, where Lupus (those born as wolves) do scoff at those Homids who consider themselves primarily human. (Sure, being too Lupus is the other end of the spectrum, also biased.)
Of course, the character sheet presents Homid/Lupus as your primary character, but that's just for practical purposes. Human players identify most easily with the human range of abilities (and wolf players with the wolf range, which is, luckily, similar to that of the humans. :)) Also, these are the two forms your character will spend most of his time - not because these are more "permanent" than the remaining three, but because he'll spend more time among the Sheep and/or the wolves of the forest than among other Garou.
...but of course I may be misremembering. It's been a long while. Others will correct me if I'm wrong. Or if there were expansions that overruled what the core book said/implied.