A quick search here or at rebornpipes.com for "pipe mud" will give you a master's thesis worth of reading on the subject. My recollection is that pipe ash is not as fine as cigar ash and is usually mixed with more coarse particles of dottle, so it's not as easy to use. There is a camp that prefers fireplace cement over the home-brew, and another argument for a complex mix of firewood ash and several other ingredients I can't recall, so your options are varied, to say the least. None of what I have to say is first-hand though, as I have not yet attempted a pipe mud repair. These are just my recollections from when I looked it up.