The fastest easiest solution is to use an external class/method to handle the decryption outside the dbUtils, and set the DbUtils.ActualConnectionString with the decrypted value.
Otherwise if you want to execute the decryption from within the DBUtils class, then you may modify the dbUtils.template, found at
<LLBLGen installation folder>\Templates\SqlServerSpecific\Net2.x\C#
If you are using SQLServer for example.
You should modify the CreateConnection method. So that whenever you re-generate the code the decryption code get generated too.
Also it might be better if you copy the dbUtils.template into a new file with another name and add your modifications there then use it in the Designer instead of the original dbUtils.template, this should keep the original template intact in case you want to revert back to it, or use it in another project.