Recent research demonstrates that naïve users can be trained to perform complex motor tasks, including trimanual activities, using an extra robotic arm (XRA). While previous studies show task‐specific improvements with XRAs, it remains uncertain whether skills acquired in one task generalize to others with differing cognitive and motor demands. This study investigates whether multitasking training enhances performance on untrained tasks involving XRA. The training combined biological functions (button pressing, slider movement, and speech) with XRA control via voluntary diaphragmatic modulation. Untrained tasks include trimanual block manipulation and concurrent block manipulation with keyboard typing. We compared performance in the untrained tasks between a group that only perform those tasks and one that completes the training beforehand. Training significantly improves performance in the trimanual block manipulation task ( t = 3.45, p = 0.001). Additionally, users’ performance in this task is significantly higher than when they used only their biological limbs, demonstrating true functional augmentation ( t = 2.70, p = 0.021). However, no differences are observed between groups in the concurrent block manipulation and typing task ( t = 163.50, p = 0.880). These findings highlight the need to explore adaptive training protocols enhancing XRA‐biological limb coordination for improved skill transfer across diverse multitasking environments.