Hi
If the dowel wood is green you can try putting the dowels in an oven or micro wave ( you'll have to experiment a bit on how long and how hot, but it should get you out of trouble. If you have any kind of table disc or belt sander you could rig up a v block jig and roll the dowels against the sander.. One final method take a piece of your dowel, screw a woodscrew into the end of it (as central as possible) leaving15mm stickng out, cut the head off ,chuck it in a drill and run it against some coarse sandpaper making a very slight taper till it's small enough to fit your holes plus a bit extra for the thickness of your sandpaper , super glue some sandpaper on the dowel and enlarge your hole to taste .
Cheers
Andrew