Hi. is there any service in wM 6.5 that we can use to write & append things to a specified file (instead of debugLog, or savePipelineToFile).
No. However, IS 6.5 does have something along those lines in the PSUtilities package that can be downloaded from Advantage and copied into your own package.
The WmSamples package also contains some file writing services to use as a starting point. This package used to be part of the IS installation, but is now separately downloadable. Although it is a “certified sample”, the same disclaimer as PSUtilities applies.