QB Issue Resolution:

No you can not, you must use the templates provided

Resolution for Issue 'Can I import my own word document invoice format into QBO?' available: Yes (Solved).
Source: Intuit Community forum.

Can I import my own word document invoice format into QBO?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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