Botium is based on WebdriverIO / Selenium - see Github Repository for the Botium Webdriver Connector. For each test case it launches a separate new Selenium session or Headless chrome session, depending on the settings.
For doing what you are trying to do there is something like a Selenium session handover required, which is not supported (and I don't know if this is technically possible with WebdriverIO / Selenium).
You can (and in most cases, you have to) add your own Javascript/Selenium scripts to do the login and initial login to the chatbot widget - see Botium in a nutshell. This will look like this example in botium.json:
...
"WEBDRIVERIO_OPENBOT": "module.exports = container.findElement('#onetrust-accept-btn-handler').then(ccBtn => ccBtn.waitForClickable({ timeout: 20000 }).then(() => ccBtn.click())).then(() => container.findElement('.troy__start-icon')).then(startChat => startChat.waitForClickable({ timeout: 20000 }).then(() => startChat.click()))",
...
(Copied from here)