Yes, you are eligible to do B.Tech from Bihar through lateral entry even though you have completed your B.Tech outside Bihar.
You are eligible for Lateral Entry Entrance Test of Bihar if you are a domicile of Bihar.
If you are not a domicile of Bihar, you will only be eligible for the lateral entry entrance test of the private universities of Bihar. The admission is done purely on the basis of the rank in the entrance exam.
If you are aspiring for Bihar Combined Entrance Competitive Examination Board, then also you are eligible to apply but the admission is subjected to the conditions of the concerned institute.