Tasks can't have their own schedules, but users can. If there is a subset of users that only work on those long-duration tasks, you could make their schedule different than the project schedule. Here are some quick notes on schedules.
If a task is assigned to a user who is associated with a schedule and resides on a project that is associated with a second schedule, you have at least 2 schedules that could potentially be applied to your timeline calculations.
Workfront uses the schedule of a user when the Calculate Resource Availability Using setting is set to The User’s Schedule in the Resource Management area of Setup.
When one user is assigned to a task, Workfront uses either one of the following schedules, as defined in the Project Preferences area of Setup:
When multiple users are assigned to a task, and the users have different schedules during the time frame of the task, Workfront uses either one of the following schedules, as defined in the Project Preferences area of Setup:
If the user assigned to the task has no schedule, or the task is assigned only to a job role, a team, or is unassigned, Workfront uses the project schedule for the timeline calculations.