Running copper to another building is generally a bad idea. Differences in the power feed or ground can cause undesired (i.e., dangerous) power flow from system to system.
Category-rated UTP (until recently, now there is ONE) not rated or certified for external use. Federal, state, and municiple code does not allow for external copper without the use of "entrance protection" at both ends to reduce the chances of dangerous current.
The ONE exception I am aware of is a product from AVAYA. They do have an external-grade Category 5 and an associated entrance protection product. The addition of the entrance protection reduces the usual ~100 meter allowance significantly.
Your best option would be fiber. It does not have the power flow problems or dangers associated with copper, and has more than enough reach for the span you need to run.
A second option would be wireless bridging.
DO NOT connect to that copper line unless you are prepared to accept the liabilities for potential destruction of the equipment and possible harmful / lethal scenario that you will create.
Scott
