You can set the configuration for dynamic content to display manual product recommendations that you decide upon and that aren't generated by any other category or parameters.
Access manual product recommendation settings by going to System > Configuration > DOTMAILER > Dynamic Content.
Setting up manual product recommendations
The products that you wish to manually push need to be entered as comma-separated product IDs under 'Products'.
Use the product picker to easily find, select and enter product IDs. Firstly, click on Choose Products to open the picker.
Upon selecting the products you want, their IDs will automatically populate within the 'Products' field.
It's useful to have fallback products ready should any products due to go into your emails be unusable, thus they can be replaced. Once again, these fallbacks need to be entered as comma-separated product IDs under 'Products'. The product picker is available here for your convenience.
Remember to click Save Config after making any changes.
Please comment underneath with any questions you may have.
Magento 2.0The above also applies for Magento 2.0, with the only difference being that the 'Manual Product Push' and Fallback Products' sections can be found by going to Stores > Configuration > DOTMAILER > Dynamic Content.