It can be done easily using regular expressions in the replace dialog.
Suppose you want to replace each occurence of two lines:
Public Property prop1() As Boolean
    Get

with the following three lines:
Something
Next line
Last line

1. Go to Edit | Find and Replace | Replace in Files
2. In the Use field select Regular expressions instead of Wildcards.
3. Type the following reg. expression in Find what:
Public Property prop1\(\) As Boolean$:Wh*Get
Note, you must escape '(' by '\(', $ stands for EOL, :WH is whitespace,
* means zero or more occurences of previous expression (whitespaces in
our case). You can find a menu ">" with other characters on the right
side of the field.
4. Type in the Replace with:
Something\nNext line\nLast line

Now you can start replace.