Is there a way, either on the Scratchpad or using a Custom List, of practicing proper names?

I'd love to drill the 姓名 of my Chinese friends and business contacts, but of course none of them are in the Skritter database.

You can add whatever personal names you like to the Skritter database. Just define them like this:

熊星汉 - Xiong2 Xing1han4 - "Xiong Xinghan (a personal name)"

丁云 - Ding1 Yun2 - "Ding Yun (a personal name)"

范大伟 - Fan4 Da4wei3 - "Fan Dawei (a personal name)"

约翰 - Yue1han4 - "John (a personal name)"

You might not remember which English name corresponds to their Chinese name, so that won't work until user-specific definitions are in (probably around the time definition practice is ready).