Be carefull going for overly long exposures. Digital sensors heat up as they record the image and long exposures can get very noisy. The idea of combining trails is a good one and reduces the chance of noise.
Even if you use Bulb to get longer than 30sec exposures, stacking is a good trick. By reducing the individual exposure times you somewhat reduce the noise in each indificual image. A bigger improvement comes from the fact that the noise pattern, being random, doesn't match image to image, and is thus suppressed in the assembly while the real image data, being aligned, is reinforced.
BTW, the Earth spins at 15 degrees per hour. To get a star trail with a small 5 degree arc you will need a 20 minute exposure (or 20 stacked 1 minute exposures). Longer arcs require longer expousres. Plan you total exposure time accordingly.