Tohei, to use a handy example, went outside of Aikido to learn some internal-strength skills and brought them back into his Aikido training.
He did not go outside of Aikido to bring the skills back into Aikido.
Koichi Tohei was born 1920 in Shitaya ward (下谷区), presently Taitō, in Tokyo. As a boy he was sickly and frail, leading his father to recommend Tohei for judo studies. He trained hard and his body prospered, but soon after he began his pre-college studies at Keio University, he developed a case of pleurisy, an inflammation of the lining of the lung cavity which causes pains when breathing. This forced Tohei to take a year off.
Tohei was distressed at the thought of losing his newfound strength of body and his means of training it, so he decided to replace his judo studies with Zen meditation and misogi exercises. As with his judo studies, Tohei entered the training of the mind with fervor and soon excelled despite his serious health issues. After his recovery from pleurisy, of which the doctors could find no trace, Tohei became convinced that it was his efforts in training his mind and cultivating his ki that had helped him to heal and recover. This stimulated his later development of Kiatsu, a system of treating physical illness by pressing with the fingers and extending the ki into the ill persons body. Tohei describes this as "priming the pump" allowing the person to heal themselves.
After recovering from pleurisy he returned to judo, but Tohei he wanted more than just physical training and did not think that judo was the right art for him to practise, although he did continue with studying judo until he started with aikido.