That seems a bit odd - "can't"? I don't see why you couldn't? Have you got a model number for the dob that someone can look up? If it's one like this, I don't see any reason why you couldnt
http://www.ozscopes.com.au/dobsonian...xon-8inch.html
It could be a focus issue, but that'd be about it. Seems to have a 2inch focuser with a 1.25 inch adapter so looks pretty normal.