I finally figured it out myself. Run help expr in LLDB and you will see:
User defined variables:
You can define your own variables for convenience or to be used in subsequent expressions.
You define them the same way you would define variables in C. If the first character of
your user defined variable is a $, then the variable’s value will be available in future
expressions, otherwise it will just be available in the current expression.
So expr int $foo = 5 is what I want.