The Bylaws of WELS address your question.
“Section 4.60 Colloquium Committee (a) Pastors and male teachers from church bodies not in fellowship with the synod shall apply for membership by written application to the president of the district within which the applicant resides. Notice of such application shall be published by the district president at least twice in Forward in Christ and/or on the WELS website and/or by other electronic means. (b) A Colloquium Committee shall be appointed by the president. The committee shall be composed of the district president of the district in which the applicant resides, a professor from Wisconsin Lutheran Seminary for the colloquy of a pastor, or a professor from Martin Luther College educational track for the colloquy of a teacher, and a vice president of the synod. The vice-president shall be chairman of the committee. (c) If the applicant is approved by the Colloquium Committee, he shall be eligible for a call, and notice of such approval shall be published by the chairman of the Colloquium Committee in Forward in Christ and/or on the WELS website and/or by other electronic means.”
A portion of a Together newsletter described one pastor’s colloquy experience.
The pastor who is interested in joining WELS would first determine in which district of WELS he resides and then contact the appropriate district president. I could provide that contact information in a private response.