Can I import data from my previous accounting software such as Reckon or even Excel, if I moved to Manager desktop?
Not the way you probably mean. Every accounting program has its own data structure. There are a few cases of claimed compatibility, not with Manager but between other programs. But forum members report even those don’t always work. You can, however, perform batch operations. See this Guide: https://www.manager.io/guides/best-practices/batch-operations-except-cash-accounts.
Manager is also designed so you do not need to carry over all your prior transactions. You only need to set a start date and set starting balances for those accounts that have them. Read some of the other Guides, which explain.