TL;DR: what's the recommended way of converting logs to a common (e.g. GELF) format?

I'm using NXLog together with Logstash and EalsticSearch. I'm collecting logs from Windows, NXLogs (internal) and my app logs using line based JSON.

Windows logs and NXLogs seem to share a lot of field names. I can write my app so that it uses the same fields. This greatly facilitates viewing data in elasticsearch. I could stick with windows fields or convert them all to GELF. AFAIK, the convertion from Windows Logs to GELF seems seems to require a lot of per-field convertion work. There is a good chance I won't get it right until enough data is produced.

I was looking for a convert_to_gelf() function which would take care of converting Windows Logs, Internal logs, IIS, etc to GELF. Is there such thing? Is manual conversion my only option?