This can only be done if the internal transformer is wound to accept either 110V or 230/220V, in which case a technician can wire it to the 230V taps. Some have this provision while most (in my experience) don't.
If you're not sure if your equipment has it, it's better to use a good quality external 230/220V to 110V transformer.
Any technician with a moderate skill level can do for you so long as you explain him what you want and don't expect any guarantee of any kind thereafter. However, I'd second awedeophile's suggestion to get a step up transformer rather than trying a tweak on the amp.
I also agree with both ! please don't bother to open and tamper with the original design ! instead buy a 220-110 converter,that'll not only do the job for you but save you the trouble and hassle of tampering the original transformer !