Margin doesn't look kindly upon inserting negative values, but TextIndent gladly accepts them. Using a combination of a negative TextIndent and a matching, positive left margin, the Paragraph is coaxed into outdenting the first line. This visually appears that the first line is even with the other paragraphs, and the following lines are indented.
<paragraph margin="25,0,0,0" textindent="-25" FontFamily="Courier"> First Line Second line Third Line </paragraph>
Produces the following:
First Line Second Line Third Line
Admittedly, this works well for the first indent, but any further indentations must be handled with a bit of creativity. I will leave you to decide how to best handle that situation.