The iminoborane (HBNH) molecule, which prefers cycloaddition reactions, selectively breaks a B=N bond of smaller diameter single-wall BNNTs and expands a ring at their surface, either at the edges or at the middle of the tube. Density functional theory (DFT) is used to test whether its organic counterpart HCCH can do the same with BNNTs. HCCH-BNNT complexes are identified and transition states located for these combination reactions. Also explored are possible reactions of HBNH with SWNTs and HCCH with SWNTs. Data suggest that B=N (C=C) bond breaking, followed by ring expansion at the surface may be possible. Although [2+2] cycloaddition reaction seems possible for HBNH-BNNTs, a high energy barrier hinders the process for other combinations of host and guest. Introduction of substituents to HBNH/HCCH may allow a facile process. In most cases of HCCH-BNNTs, HBNH-SWNTs, and HCCH-SWNTs, transition states are identified and suggest an electron-rich guest might lower barrier heights to form stable complexes. Reaction with HCCH or HBNH at the bay-region of smaller diameter armchair tube is not favorable.