Just enter in an IDEALPRO currency order. For example, if you have USD and want JPY, enter a sell USD.JPY order. You'll end up with a positive JPY balance and a reduced (or negative) USD balance. Or buy USD.JPY if you want to go from JPY to USD.
Note that this might set up a virtual position in TWS but this can be cleared out by right clicking on the virtual position in the account screen and selecting the option to change the position. Set it to zero and it will disappear.
IB does not do these conversions for you except when there's a very small balance (e.g. < $1). These they periodically clear out for you. So if, for example, you buy a stock listed on the TSE, you will have a CAD margin loan unless you convert USD to CAD by selling USD.CAD.