Inserting bytes in the middle of binary file

Question

I want to add some string in the middle of image metadata block. Under some specific marker. I have to do it on bytes level since .NET has no support for custom metadata fields.

The block is built like 1C 02 XX YY YY ZZ ZZ ZZ ... where XX is the ID of the field I need to append and YY YY is the size of it, ZZ = data.

I imagine it should be more or less possible to read all the image data up to this marker (1C 02 XX) then increase the size bytes (YY YY), add data at the end of ZZ and then add the rest of the original file? Is this correct?

How should I go on with it? It needs to work as fast as possible with 4-5 MB JPEG files.