Yes. Some bacteria have D-amino acids (such as D-alanine) as part of their cell walls (which otherwise contain almost entirely L-amino acids). D-amino acids are also sometimes incorporated into drugs that are synthetic peptide mimics in order to slow metabolism.
You can make these things in a lab. However Phind said "In summary, while scientists have proposed various models for how biological homochirality may have emerged, there is currently no known example of mixed protein chirality occurring in nature. Biological molecules appear to exhibit almost exclusive homochirality at the molecular level."
It makes sense, I guess. Why would something natural want to interact with both chiralities of a target? Usually there's a reason for one... and the other is unrelated.
it is possible to have mixed chirality in syntheic proteins. (see michael weiss work investigating insulin receptor binding to insulin)
its basically impossible (but not totally impossible) for a living creature to be able to generate any protein with mixed chiralities.
this is because a ribosome with a chamber that can support both chiralities is likely to be less efficient at protein extension. but also you need so much more trna if you want to support arbitrary d-amino acids, etc.
Not sure what you mean by "in a protein", but if you have a solution of some chiral chemical compound such that there's 50% of the L-enantiomer and 50% of the R-enantiomer, you get what's called a "racemic" mixture. So, yes -- mixing chirality is possible in at least one sense.