This article presents the preparation, compositional and electromagnetic characterization of modified few-walled carbon nanotubes/nanofibrillar cellulose (FWCNT/NFC) aerogels integrated in a standard terahertz hollow waveguide and studies their operation as absorbers of electromagnetic waves in the WR-3.4 band (220-330 GHz). Hybrid aerogels consisting of different weight ratios of NFC and modified FWCNT are prepared by freezedrying and characterized through scanning electron microscopy and Raman spectroscopy, and then placed within waveguide cassettes in a simple, low-cost and efficient way that requires no special equipment. A broadband measurement setup is employed for examining the electromagnetic response of the materials. It is found that the materials are excellent absorbers with an average shielding efficiency of 66 dB in the best case and return loss above 10 dB across the band with a flat frequency response. FWCNT aerogels are assessed as a promising candidate for terahertz waveguide terminations.