Not currently. unless you modify the Rockbox source code to behave that way and compile your own version of Rockbox (which will require a little C programming skill).
But there was discussion in IRC yesterday of introducing a new setting for USB insertion - the idea (if I've understood correctly) is that the setting would have three values - "Connect, Charge and Ask". If one of the first two is chosen, then the idea is that a user can hold any button whilst inserting USB to reverse the behaviour. If Ask is chosen, then a dialog will appear asking the user if he/she wants to either connect or charge.