You're going to have to do some custom css rules, that override all existing toolbar-style button rules in place. Also, you will have to get some custom images (normal, hover and clicked).
It will probably be to your advantage to determine what ExtJS can do easily, and then "sell" your users on how much cooler that is than whatever they are asking for.
There usually isn't a return-on-investment (for you, yes, but mainly for them, given that they're paying you to do this...) in trying to give them exactly what they're used-to from some other application. (And they might be asking an innocent request, with no knowledge of what the request might actually cost ... and no way to know, of course.) But, people are very adaptable.