You don't usually want to mess with splicing USB cables. Data transfer might be messed up when the cable's shielding and parallel wires have a break - especially in highspeed mode.
instead you can use commercially available adapters, as they ship with a lot of peripherals, especially HDDs but also other power greedy stuff. Similar to this:

The silver-transparent adapter came with the white 3G-Dongle. It is a USB-AM<->USB-AF extension with a branch to an auxiliary 2nd USB-AM from the first one. The 2nd plug is only for powering VBUS, and is supposed to get plugged into a second host port to supply the 3G stick with an additional 500mA. Here it's plugged to a USB wallwart charger, thus powering both ends of the whole cable nest with 5V. You can use such a setup right now for powering the peripheral so N900 doesn't have to provide the current - just plug in the N900 side of cable immediately after you clicked "VBUS boost on" in h-e-n GUI. The N900 will not mind any external supply supporting the internal 5V booster.
see http://talk.maemo.org/showthread.php...203#post921203 for powering the N900 while in hostmode.

cheers
jOERG