How to append the data into a single cell of an existing csv file

@Loganayaki
I've edited your OP using markdown code tags to make it clearER.
Please try using the markdown code tags going forward.
As far as the requirement goes... Yes, CSV can potentially have multiline fields - although I don't know if it's part of the RFC or not.
I'll need to dig up my cliff notes from the previous life when I had to parse them (when I have time).

Could you provide a small representative sample input and a desired output, please.
Use markdown code tags as previously requested.