You will have to add them manually.
hmm, you mean draw the lines manually, and remove the underline setting from the paragraph style? Rats!
No just add a series of Underline characters to the last line to fill it out.
Position a right-aligned tab stop where you want the underline to end. Save it as a Paragraph Style.
Or, set the text as right-aligned, then type a tab character after the text to force it left. The rule will then extend to the right edge of the textframe.