Even if the block is locked we can add a paragraph before it.

common/Private/Locks.js

@@ -625,6 +625,9 @@ CTable.prototype.CheckContentControlDeletingLock = function()
 };
 CBlockLevelSdt.prototype.Document_Is_SelectionLocked = function(CheckType, bCheckInner)
 {
+	if (AscCommon.changestype_Document_Content_Add === CheckType && this.Content.IsCursorAtBegin())
+		return AscCommon.CollaborativeEditing.Add_CheckLock(false);
+
 	var isCheckContentControlLock = this.LogicDocument ? this.LogicDocument.IsCheckContentControlsLock() : true;
 
 	var nContentControlLock = this.GetContentControlLock();