you can connect real buttons to the digital inputs using the diagrams for buttons you find here https://learn.bela.io/tutorials/pure-data/connecting/digital-input/ and that will make it available as a digital input in the Bela program.

To use the digital input inside rnbo, follow the instructions here https://learn.bela.io/using-bela/languages/rnbo/#assigning-bela-digital-inputs-to-a-parameter