Can I use hamster food (a seed mix that has raw wheat, oats, barley, rye spelt) like this for Pesach?
Why should it be okay? We cannot assume that they didn’t use water. The only question would be if it were fit to be eaten by humans (ignoring the “gross” factor because they know it is hamster food). If it were never fit for humans, then it is chameitz nukshah, so one does not transgress de’orayta of bal yera’eh, but it is still assur derabanan to keep. See Mishnah Berurah 442:2. If it is not fit for achilat kelev, then it is permitted, but I don’t think you can argue that this food isn’t fit for animal consumption! So – short answer – you are not allowed to own it on Pesach. If there are no alternatives, perhaps we could explore having a non-Jew own it and you asking to use it to feed to your hamster. Let me know if you want me to address that case.
If one were to inquire of the manufacturer regarding whether or not water is used?
I’m not sure you could get a sufficiently reliable answer as to whether any of the components came in contact with water at some stage during the process. And then there is the process of tempering – see here.