Can resistance be added to speakers to make a 4 ohm speaker have an 8 ohm load?

You "can" add a four ohm resistor in series with a four ohm speaker, and make it look like an eight ohm speaker, however, the frequency response will not be the same. It is better to use to correct speaker for the amplifier, or use two four ohm speakers in parallel.

That resistors power rating must exceed the rating of the speaker. Half the from the amplifier will go to the speaker, while half will go to the resistor, making it get quite hot.