Variable / String Appends in Powershell

I'm in the process of building a PowerShell script which is making heavy use of ConvertTo-HTML to serve as the basis for a web-based reporting tool.

However, I've hit a brick wall with regard to how variables may be appended.

The script sometimes breaks out into a loop when multiple databases need to be interrogated. For  the first database, the HTML is built for the result set as a variable with the last script action being "ConvertTo-HTML  xxxxxx | Output-String".

For the second and subsequent databases, I simply want to append the output to the existing variable.

Any pointers greatly appreciated!