Insert commas into a row/cell of words

Background info: I have access to Excel 2007. I am using it because I need to move data from one website another website and the content management systems will allow export/import using csv files. Everything is okay except for one column holding tags in the exported file.

The tags for each piece of content were exported into a column of rows like so:

word1 word2 word3
word1 word2
word1 word2 word3 word4
word1 word2
word1

The import function on the second site expects the tags to be separated by commas. I did a bit of reading online but the examples I've found don't help my situation because a lot of the tags are not single word tags. Doing a simple replace of spaces with commas would not produce the correct data.

I can get an export of the tags separated into one per row like so:

tag1
tag2a tag2b [this indicates a multi-word tag]
tag3
tag4a tag4b tag4c
tag5a tag5b
tag6
etc.

I am wondering if an excel function could look at each row/cell of tags separated by spaces and look at the full list of separate tags and then insert a comma after any single/muti-word match found so data would look like so:

word1 word2, word3
word1, word2
word1 word2, word3 word4
word1 word2
word1

I've attached an example of what I have in mind. Sheet 1 is how the data currently looks, sheet 2 would be the list of tags and sheet 3 is how the data should look after the commas are inserted.

@shg - tags and words are the same things, tags are made up of single word or multiple words.

Yes, nothing is being changed on the old site so the data is static.

Sheet2 is a list of all the tags attached to all the content. Each row in sheet1 represents one piece of content and the specific tags for that piece of content from the list in sheet2.

Thanks for the help. I tried this out on the sample workbook and it worked like a charm. I will try it out on the actual data. Hope it works just as well on that too.