Can I convert my 27speed tourer?
 

I want to convert my Venture GT 27speed tourer to an internal hub geared system,or failing that to a fixed wheel type,can it be done,and would it be very expensive?

Sheldon Brown's web site is a treasure trove for both fixed/single speed and internal gear hub conversions.

Depending on the hub you select, you will have to 1. accommodate the cable to the hub, which may be either to the right chainstay or the left, depending on the hub you choose and, 2., you will have to find a way to properly tension the chain once the hub is installed. Tensioning can be done by a jockey wheel spring tensioner, as with a derailleur, or by a horizontal dropout (which may mean having new dropouts brazed in--the Venture GT is Reynolds 725 steel, I believe) or by an eccentric bottom bracket. The jockey wheel would be the cheapest but "dirtiest" solution.

Costs: hub of choice, with cables, and shifter, barend or other mount for a shifter (probably would have to be mounted to your stem), costs of wheel rebuild, costs of jockey wheel tenisoner, costs of new chain.
Alternatively, delete the jockey wheel, add the brazing and costs of bottom bracket replacement and respray work or add the costs of an eccentric bottom bracket. The eccentric BB would be second cheapest.

Having a new hub laced into the current rim you have, that being the rim is in good shape, and adding a tensioner isn't that expensive and much less labor intensive then having a new BB shell welded or brazed onto the frame. Shimano has freewheeling 7 speed rear hubs, allowing you to use your current brake setup. Installing a shifter isn't that big of a deal, although attaching the cable into the rear housing can be a PITA.