Currently there is no option on the system or group level to set the tick for "Keep the planned completion date of the issue" when converting an issue to a task. We have the option to set defaults for "Keep the original issue and tie its resolution to this task" and "Allow Primary Contact to have access to this task", but not for the last option. Even in the documentation this feature is listed as possible:
"Keep the planned completion date of the issue
If unselected, the Planned Completion Date of the new task is calculated from the Planned Start Date of the task. The Planned Start Date of the new task is set according to the system preferences for new tasks.
Or, if the top-level groups in your organization configured them separately, the options that display here depend on which group is associated with the project you selected in step 6. For more information, see Configure task and issue preferences for a group."
A default setting would be very helpful and time saving for setups where a lot of requests/issues are converted.