The best way to achieve this is to follow the steps below:

• Create a new Extra field in Central, called something suitable such as 'CRM bulk sync' and set it as a text field, ensure it is available to clients and contacts.
• Create an equivalent custom field in CRM to receive the data.  There's no need to add it to a form.
• In Central, go to File > Maintenance > CRM > Extra field mappings.
• For both person and organisation records, map the two fields so that any data from Central will go into the CRM field.
• In Central, search for all contacts and/or clients that need syncing and tick the left hand column to select them all (consider doing an advanced search to exclude those with the Extra field 'exclude from sync' set to Yes).
• From the ribbon click on Extra fields.
• Add some text to the value column (just a single letter will suffice) for each client.  You can use Ctrl C and Ctrl V to quickly copy and paste the value in all the fields.
• Click Apply.

The next time your CRM syncs with Central this item of data will be sent across, along with all other data from those selected clients and contacts, thus achieving a bulk re-sync.