diff --git a/src/data/bedrock/LegacyToStringBidirectionalIdMap.php b/src/data/bedrock/LegacyToStringBidirectionalIdMap.php
index 2ffe1b986..e39df6ad9 100644
--- a/src/data/bedrock/LegacyToStringBidirectionalIdMap.php
+++ b/src/data/bedrock/LegacyToStringBidirectionalIdMap.php
@@ -81,4 +81,15 @@ abstract class LegacyToStringBidirectionalIdMap{
 	public function getStringToLegacyMap() : array{
 		return $this->stringToLegacy;
 	}
+
+	public function add(string $string, int $legacy) : void{
+		if(isset($this->legacyToString[$legacy])){
+			throw new \InvalidArgumentException("Legacy ID $legacy is already mapped to string " . $this->legacyToString[$legacy]);
+		}
+		if(isset($this->stringToLegacy[$string])){
+			throw new \InvalidArgumentException("String ID $string is already mapped to legacy ID " . $this->stringToLegacy[$string]);
+		}
+		$this->legacyToString[$legacy] = $string;
+		$this->stringToLegacy[$string] = $legacy;
+	}
 }
diff --git a/src/data/bedrock/block/upgrade/LegacyBlockStateMapper.php b/src/data/bedrock/block/upgrade/LegacyBlockStateMapper.php
index 0822fe817..728ef05b5 100644
--- a/src/data/bedrock/block/upgrade/LegacyBlockStateMapper.php
+++ b/src/data/bedrock/block/upgrade/LegacyBlockStateMapper.php
@@ -52,6 +52,18 @@ final class LegacyBlockStateMapper{
 		return $this->fromStringIdMeta($stringId, $meta);
 	}
 
+	/**
+	 * Adds a mapping of legacy block ID and meta to modern blockstate data. This may be needed for upgrading data from
+	 * stored custom blocks from older versions of PocketMine-MP.
+	 */
+	public function addMapping(string $stringId, int $intId, int $meta, BlockStateData $stateData) : void{
+		if(isset($this->mappingTable[$stringId][$meta])){
+			throw new \InvalidArgumentException("A mapping for $stringId:$meta already exists");
+		}
+		$this->mappingTable[$stringId][$meta] = $stateData;
+		$this->legacyNumericIdMap->add($intId, $stringId);
+	}
+
 	public static function loadFromString(string $data, LegacyBlockIdToStringIdMap $idMap, BlockStateUpgrader $blockStateUpgrader) : self{
 		$mappingTable = [];