Hi Parin,
Sorry about the delay. Your email went to my junk folder. I know we did a simple generic sample over a year ago, but I couldn't find it just now. So, here's a description...
Assume field1 is a dropdown. Your InfoPath form will have two sections - one with that dropdown and another with a text box which is bound to the same field1. By default, the second section is hidden. To do this hiding, add a True/False (Boolean) field to your data source (hereafter called "field2"). You will have two buttons - one in each section. The first button in the section with the dropdown can have the label "other". Clicking on that causes a rule to run that sets field2 to false and then you have conditional formatting rules that hide the dropdown section if the field is not true. Another button in the text box section is called "save" and this button will copy the new value to the SharePoint list, re-query the secondary data connection to it, and set field2 to true so that the dropdown is once again displayed.
To show how to create the value in the SharePoint list you can use either CAML or qRules SubmitToShPList. It's relatively easy but will take between 30-60 minutes to configure the first time (after you get the hang of it, it will be easier). The CAML technique was described in Jo-Anne's webinar on January 31st of this year and it shows how to map InfoPath form data to a SharePoint site's Tasks list. Same technique can be used for any list. Doesn't have to be the tasks list. The technique is also described in the Dashboard webinars from February 28th and March 7th. To watch these webinars please navigate here: http://www.qdabra.com/en/training/trainingvideo.aspx and click on the webinar links. For a very small processing fee you can purchase the samples we showed in the webinar. We provide Qdabra Webinar Kits for them. For example, here's the link for the Tasks one: http://www.qdabra.com/webstore/p/61/qdabra-webinar-kit-task-xtp-for-infopath-integration-with-sharepoint-tasks.
We can also do this for you, but since it could take up to an hour, we kindly request a Qdabra Support Pack. Let us know if you would prefer that option.
Good luck!