No, Dorel does not allow the use of either the tether or the lower anchors while in booster mode. Do you have your seat manual? There ought to be instructions for stowing those parts.
I will caution you about using that seat as a booster at all. It typically provides very poor lap belt fit by positioning the lap belt high on the belly where there are no bony parts to protect the internal organs, and often poor shoulder belt fit (depending on the child's height). Is a dedicated booster an option at all? Booster fit varies dramatically by vehicle and child, and is all to do with the geometry of the seat belt and shape/build of the child. Usually you can get superior belt fit with a dedicated booster seat, and if using the lower anchors is appealing to you there are boosters that come with that feature in order to avoid having to rebuckle the empty seat. Optimal belt fit is key with boosters, because if the belt isn't fitting properly, the child isn't well-protected in a crash.
If a dedicated booster is a possibility, or you'd like help determining how the belt is positioned on your child with that seat, please post a photo (blur the face if you feel more comfortable doing so).