This question I have passed on to several servants of Christ with experience in ministering to Muslims and to Christians from a Muslim background. It turns out that the difference of opinion in your church in Nigeria is also there among the different brethren that I asked. So I am going to first send you the answers that I received. Then at the end I will give some additional background information on this issue. The six answers listed below range from extensive apologetic / doctrinal arguments (Answer 1), through detailed pastoral experiences (Answers 2 + 3) and ending with short statements that summarize suggestions practically (Answers 4,5 and 6). The first four answers address your specific case, whether a polygamous convert can exercise an office in a church.