Can I sign a contract through other platforms without patent agency qualification?

Of course. According to Article 25 of the Regulations on Patent Agency in People's Republic of China (PRC), units and individuals that have not obtained the qualification of patent agency business or engaged in patent agency activities beyond their business scope shall not publish, solicit, undertake or implement patent-related business in any form. Therefore, they can't directly carry out this work before obtaining the corresponding qualifications, but they can still seek suitable resources on other platforms, such as signing contracts through other platforms to help complete related tasks.