According to Peking University Master's Admissions Guide for 2023 (Headquarters), we can know that:

Persons who sign up for the entrance examination for master of laws in Peking University shall meet the following conditions:

(1) meets the requirements in article 1 of the national unified examination (1) conditions for entering the examination.

(2) Candidates who apply for the Master of Laws (Law) major must be law majors before applying for the exam (only graduates of law majors (code 030 1) in the undergraduate professional catalogue of ordinary colleges and universities, graduates of law majors at the junior college level, graduates of law majors in the form of self-study exams, and those who have obtained the second bachelor of laws degree can apply for the exam).

It can be seen that LLM enrollment in Peking University does not accept cross-exams, which is a hard and fast rule.