Like Dspark, I am having some difficulty in getting the picture of your application. Sounds like your Service Entrance is coming out of a Utility transformer underground to a light pole, possibly for a light fixture. Now you want to tap the SE, and continue to a dwelling. Taking the liberty to make these assumptions, i think the tap would want to be the same size as the SE, assuming it is properly sized. If it is SE cable per Art. 338, then the box may need resizing due to the bend requirements. Then the splice will have to be properly done in accordance with Art 110-14.
The best way would be as you suggested, a continuous SE from Utility to dwelling, then to feed the light from there.