Hi @ppcm, welcome to the community!
I haven’t used the n8n-nodes-fixed-width-file community node myself, but from looking at your screenshot it appears you have a simple binary object in n8n. To manipulate the content you’d first need to make it available to n8n using the Move Binary Data node. You can then use a Set node to wrap lines around the existing content.
Here’s how this looks in a workflow:
Input file:
Result file:
Hope this helps!